I. Field of the Invention
The present invention relates to a starting circuit arrangement for gaseous discharge lamps, and more particularly to a circuit arrangement for starting a linear metal halide arc discharge lamp while the lamp is in a semiconductor inverter circuit.
II. Description of the Prior Art
Gaseous discharge lamps of the long, linear, multicomponent, metal halide type are extremely susceptible to cataphoretic effects. That is, the color of the radiated light may vary along the length of the lamp due to the influence of thermal and electrical gradients and the like which act to produce a non-uniform dispersion of light emitting cations in the arc discharge lamp. Light feedback, closed-loop, electronic lamp current switching circuits have been devised to counteract these non-uniform dispersions. By controlling relative time of forward to reverse alternations of current in the lamp, it is possible to inject a DC current (superimposed on the AC current) which produces a cataphoretic bias of its own and can be poled so as to oppose the naturally occurring cataphoretic forces. It is possible to construct a circuit operating in such a negative light-feedback control system as described in U.S. Pat. No. 3,700,960 -- Lake, assigned to the same assignee as the present invention, which will produce a uniform, axial, spectral energy density of light output for such lamps. One of the greatest difficulties facing designers has been in finding circuits which will reliably start such lamps without degrading or causing failure of the electronic switching apparatus needed to control cataphoresis as described.
In addition to being susceptible to cataphoresis effects, multicomponent metal halide lamps are difficult to start. The application of a very high ionizing voltage is required to initiate breakdown leading to a continuous arc discharge. It has been found, however, that, in pulse starting, crest voltages of 25 Kv with a typical rise time of one microsecond are required. This is not surprising since these are the typical parameters for pulse starting other long linear arc discharge lamps of similar dimensions. U.S. Pat. No. 3,700,960 -- Lake discloses a metal halide lamp system; however, an arrangement as disclosed therein in FIG. 5, a Tesla coil was used as the source of high voltage for initiating breakdown of the linear metal halide lamp. A plurality of bypass switches shown in FIG. 5 thereof and totaling six in number are incorporated in circuit with the lamp 1 between DC input 11 and the circuit ground connection. These bypass switches are a set of mechanical contacts which serve to transfer aside and isolate the comparatively delicate semiconductor switching inverter switches from the deleterious effects of the high voltage starting pulses applied to the lamp during lamp starting.
Typically, the pulse starting voltage is 10 to 50 times greater than the blocking voltage rating of the highest voltage-rated power transistors suitable for each leg of a bridge inverter such as that disclosed in the aforementioned Lake patent. One can easily see that the destruction of these transistors would be the general consequence of attempts to start the lamp were it to be directly connected to the bridge inverter without special protective provisions.
It is desirable therefore to provide a starting circuit arrangement for a gaseous discharge lamp, for example a linear, multicomponent metal halide arc discharge lamp, while the lamp is in a semiconductor switching inverter circuit which is in the running inverter mode.